Electric protective system



Jan. 2, 1951 J w ss 2,536,809

ELECTRIC PROTECTIVE SYSTEM Filed July 20, 1949 Inventor:

James W l-lodekiss Hi S Attorney Patented Jan. 2, 1951 UNITED STATES PATENT OFFICE ELECTRIC PROTECTIVE SYSTEM Application July 20, 1949, Serial No. 105,788 In Great Britain August 26, 1948 4 Claims.

This invention relates to electrical unbalance protective systems and more particularly to a protective-relaying system of the pilot-wire type wherein direct current is injected into the electrical unbalance pilot-wire circuit for controlling the circuit interrupters located at a plurality of the terminals of an alternating current electric power system.

In known electric systems of the pilot-wire type, a protective relay is located respectively at each terminal of the protected line section and such relays are interconnected by pilot wires in order to provide for operation in response to unbalanced electrical conditions. Under normal load conditions or when a fault external to the protected section occurs, the power-system current flowing through each terminal bears a predetermined relationship to the current flowing through the other terminal or terminals of the system. Electrical quantities derived from the power-system current at each terminal, when supplied to the pilot-wire network, can be arranged in a balanced relationship so that no appreciable current flows in the pilot wires when an external fault occurs. When an internal fault occurs, an unbalanced condition develops between the electrical quantities derived at the various system terminals. This unbalanced condition in known arrangements will cause operation of the protective relay located at at least one of the system terminals thus causing disconnection of the associated circuit interrupter. The protective relay at another of the terminals of the protected section might not operate because the current flowing through the system conductors at such terminal may be too small to cause operation of the corresponding protective relay. Accordingly, it has been the practice to utilize the protective relay which operates during an internal fault for the purpose of injecting a direct current into the pilot wires. This injected current causes operation of polarized receiving relays located at the terminals Whose protective relays do not operate. Operation of the receiving relays causes tripping of their associated interrupters. Since the current injected is direct current, it is necessary to use two polarized receiving relays at each terminal so that response can be obtained for direct current injected into the pilot wires from either of two directions in presently known arrangements.

It is an object of my invention to provide a protective system of the pilot wire direct current injection type which utilizes only one receiving relay at each of the system terminals.

Another object of my invention is to provide a protective arrangement for pilot wire protective systems of the direct current injection type which is applicable to a protected section having any number of terminals.

In accordance with my invention, I eliminate one or" the direct current receiving relays nor mally required in arrangements of the direct current injection type and provide a protective system which can be used for any number of terminals by connecting the operating windings of the polarized receiving relays in series circuit relationship so that a direct current of predetermined direction of flow which is injected into the line at any terminal will cause simultaneous operation of all of the receiving relays.

One form of my invention is embodied in the three terminal system represented by the single feature of the drawing. For purposes of simplicity, I have shown a single phase power system but it will be understood that my invention is equally applicable to a poly phase power system.

With reference to the drawing, the protected portion of the system comprises the line sections l and 2 arranged in a T-connection. Line section i interconnects buses 3 and 4. Circuit breakers 5 and 6 are provided for isolating line section i from the buses 3 and 4. The circuit breaker l is for the purpose of disconnecting line section 2 from its source or feeder line. Circuit breakers 5, 6 and l are shown schematically as being of the biased-opened latch-closed type which are operated respectively by trip coils 8, 9 and 10.

The protective system associated with the terminal controlled by circuit breaker 5 comprises a fault responsive protective relay Ha, a polarized receiving relay 52a and an injection relay Etc. The corresponding relays associated with the line terminals controlled by circuit breaker 6 are given the designations H1), 121) and it while the corresponding relays associated with the line terminal controlled by circuit breaker l are given the designations Hc, I20, and E30. Since the relays and their circuits are the same for each terminal the same numerals will be used to designate the corresponding components. The letters a, b and 0 will be used in conjunction with the appropriate numeral to designate the components for the terminals controlled by interrupters 5, 6 and '5 respectively. Relay 5 la is provided with a primary winding Ma which is energized by current transformer Etc, the primary or" which is connected in series with one of the conductors comprising line section l. The clay Ela is provided with a secondary winding i611. As will be more fully explained hereinafter, the windings a, lfib and lfic are all connected in series circuit relationship with the voltages generated therein in opposition. Under normal conditions, no appreciable amount of current flows in the windings lea, Ito and lSc or in the pilot wires which interconnect these windings. An unbalanced condition, such as could be caused by an internal fault, will cause a current to flow in the circuit comprising the windings lta, i517, c and the pilot wires. Relay Ha is also. provided with an element Ha which is movable when a predetermined current flows through the windings its and 94a. Movable elementilla controls contacts Illa of the relay 1 la.

The injection relay 13a is operable in response to operation of the contacts Illa of faultresponsive relay i la. Associated with the injection relay l3a is a source of direct current potential indicated at Ida and in series wilh the battery la is a current limiting resistor Eda. The relay l3a is provided with normallyopened contacts 2 la and normally closed contacts 22a and a suitable arc inhibiting device may be connected in parallel circuit relationship with the contacts 22a as indicated at 23a.

Trip coil 8 is operated when relay lla operates because of the completion of a, circuit comprising the positive terminal of battery lea, resistor 253a, the coil of relay l3a, contacts lac of relay Ila, trip coil 8, conductors 24a and a and the negative terminal of battery lea. Thus, operation of relays l la, lib and lie causes opening of circuit breakers 5, 6 and '3 respectively.

Since a fault condition which is internal may cause suilicient current to flow through the wind ing Mb of relay lib for example, but not enough current to flow through the windings Mia and i of relays Ma and lie to operate the relays i la and No, it is necessary to provide means for positively opening circuit breakers 5 and i when circuit breaker 6 is'caused to open. To this end, each of the terminals is provided with a polarized receiving relay such as 92a, lZb and Me which will'operate in response to operation of i any of the injection relays such as relays i366, 13b and I30. Belay 52a is provided with an operating winding 'Ztaand a restraining winding Zia. Operating winding Zea is arranged in series with a suitable choke coil 28a. Because the pilot I wire may receive a high induced voltage due to the proximity of the protected line sections l and 2, for example, the choke-coil 28a is effective to immunize winding 23a against alternating or surge currents. The restraining winding Zia is energized by a rectifier arrangement Eric, the alternating current terminals of which are connected in parallel with the operating winding 25a and its associated choke coil 28a. Direct current output of the rectifier 29a is supplied to the restraining winding E'ia. telay 52a is provided with norma ly opened contacts 39a which are closed in response to the flow of a predetermined direct current through the operating winding 2511..

Each of the receiving relays is effective to cause tripping of its associated circuit breaker. For example, relay its will close its contacts 38c when an appropriate direct current signal is received from battery -!9a or lsb. Closing of the contacts 380 of the relay l2c places battery ltc across the trip coil ill by completing a circuit through conductors 25c and 260, contacts 389, the coil of relay 13c andresistor Zilc to cause a tripping operationof circuit breaker 1.

From thedrawing, itcan beobservedthat the 4- relays Ila, l2a and Ho are connected with the corresponding relays associated with the terminals controlled by circuit breakers 6 and I by pilot wires 3!, 32 and 33 which form a series circuit into which the batteries 19a, I92) and ISc can be inserted upon operation of any one of the protective'relays and the resulting'operation of the associated injection relay. The polarity pf the batteries l 9a, H31) and I is such that their voltages would be additive so that only one polarized receivingurelayneed be used at each terminal. The

resisto1's"2Ela,'2llb and 260 are chosen with resistance values suchthat the voltage would not be excessive should more than one of the batteries l 9 beinj'ected intothe circuit at one time.

Should afault condition develop on line section vi, for example, which would cause operation of protective relay Ila but which would not cause operation of relays lib and No, the coil of relay l3a would be energized upon closing of the contacts' i8a. Since a circuit would be completed between the positive terminal of battery Iila, resistor Zila, the coil of relay l3a, contacts i811, trip coil 8 and conductors 24a and 25a as already explained. This would result in opening of circuit breaker 5 andclosing of contacts 2 la of relay l3a. Upon closing ofcontacts Zia, a circuit would be completed through the coil lea, contacts)? la, re-

sistor 28a, battery i911, conductor 25a, operating windingzzfia of.-r elay 12a, pilot wire 32, coil lfib of relayzl Eb, contacts 22b of relay l3b, conductor 2%, coil 26b of relay izbypilot wire 33, coil H50 of.relay=l lc, contacts 220, conductor 25c, coil 26c,

and pilot wire 3! to the coil lfia of relay Ila.

This series circuit would cause operation of relays l2b andlZc-andclosing of their contacts Bllb and 39c would energize the trip coils 9 and ill respectively. Thus, operation of relay i la would, for example, cause operation of relay lfia which in turn would cause batterylSa to inject a direct current potential in a series circuit comprising the operating windings 26b and 25c of receiving relays 'iZZrand lZc. .Energization of these windings would result-in the opening of circuit breakers 6 and i almost immediately after opening of-circuit breaker 5 upon the occurrence of an internal fault within theprotected portion of the power system. Likewise, operation of relays l lb and He would cause operation of interrupters 5, r

5 and ll.

Because the polarized receiving relays l2a, 12b and lZcare-connected inseries with each other and because the potentials of the batteries [90,, lab, and are additive, it is only necessary to use onereceiving relay at each of thesystem terminals. protective arrangement comprising my invention is notlimited to three terminals as shown but could be extended to-an indefinite number of terminals.

While I have shown and described a particular embodimentof my invention, it will be obvious to those ski led in the art that various changes and modificationsmay be made without departing from my invention in its broader aspects and I, therefore, intend in the appended claims to cover'all such changesand modifications as fall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patentof the United States is:

1. A protective arrangement for an alternating current electric-power system having a plurality of terminals comprising an electrical unbalanced protective-relay at each'terminal having an alternating current response windingconnected in a Furthermore, it willbe obvious that the i series pilot circuit with the alternating current response windings of the protective relays at the other terminals for performing a predetermined control operation when an unbalanced electrical condition develops, a polarized receiving relay disposed at each terminal end of the system for independently performing the predetermined control operation of the corresponding protective relay, each of said receiving relays having an operating winding, means including a rectifier for interconnecting the operating windings of said receiving relays in the series pilot circuit to be unresponsive to alternating current therein, a source of direct current potential at each terminal, and switching means controlled by the unbalanced protective relay at each terminal for inserting the corresponding source of direct current potential in the series pilot circuit to energize the operating winding of all of said pOlari' ed receiving relays upon operation of the unbalanced protective relay in response to the occurrence of an unbalanced alternating current condition within the system.

2. A protective arrangement for an alternating current electric power system having a plurality of terminals comprising a polarized receiving relay at each terminal of the system for performing a predetermined control operation, each of said relays having an operating winding, means for interconnecting the operating windings of said relays in a series circuit, an unbalanced alternating current responsive relay at each terminal for performing the predetermined control function of the corresponding polarized receiving relay when a predetermined alternating current unbalanced condition develops and each having an operating winding connected in the series circuit, a source of direct current potential at each terminal, and switching means controlled by the unbalanced alternating current responsive relay at each terminal upon response thereof for inserting the corresponding source of direct current potential in the series circuit for supplying current to the operating windings of all of said polarized relays.

3. A protective arrangement for an alternating current electric power system having a plurality of terminals comprising a polarized relay at each terminal of the system for performing a predetermined control operation, means including a two-wire pilot circuit for interconnecting said polarized relays in series, an alternating current unbalanced responsive relay at each terminal of the system having an operating winding connected in the two-wire pilot circuit, a source of direct current potential at each terminal of the system, and switching means controlled by each alternating current unbalanced responsive relay upon response thereof for inserting the corresponding source of direct current potential in the two-wire pilot circuit for causing a direct current to flow in a predetermined direction through all of said polarized relays.

4. A protective arrangement for an alternating current electric power system having a plurality of terminals comprising a polarized relay at each terminal of the system for performing a predetermined control operation, an alternating current unbalanced responsive relay at each termi nal of the system operable in response to a predetermined system unbalanced condition, a two- Wire pilot circuit interconnecting predetermined windings of said polarized relays and said unbalanced responsive relays in series circuit relationship, a source of direct current potential at each terminal and an injection relay located at each terminal which is controlled by the corresponding one of the unbalance responsive relays and provided with switching means for inserting the corresponding source of direct current potential in the two-wire pilot circuit to cause a direct current to flow in a predetermined direction through all or" said polarized relays in response to the occurrence of the predetermined unbalanced condition within the system.

JAMES W. HODGKISS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,849,830 Gross Mar. 15, 1932 1,950,193 Ross Mar. 6, 1934 2,096,614 Lehmhaus Oct. 19, 1937 

